eli5 Noise cancelling headphones are magical but how does the technology work and does it drain the battery?
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It works by having a little microphone right next to the speaker listening for the outside noise that you want to cancel, then playing the inverse of that signal into the speaker, such that the two sound waves should cancel each other out. It can drain the battery more quickly than without since it needs to be doing active processing of the microphone signal to do this.
I’m assuming you mean active noise canceling. You’re in luck because I’m autistic and recently looked into this.
So your headphones have a microphone that “listens” to the background noises and basically plays equal but opposite soundwaves to cancel out the sound. Think about it like this, the soundwave it’s detecting is +2. Your headphones will then play -2. So you’re hearing 0.
Does it protect your hearing from continuous machine noise that isn’t too loud?
First off, there’s a big difference between noise cancelling and noise suppression. Suppression works by blocking the outside sound from getting in. This is a passive process. Noise cancelling is an active process, and runs the battery down quicker.
You can think of sound as the air wobbling up and down. (Really, it’s getting squashed and stretched, like thrusting one end of a slinky, but let’s keep it simple to visualise it.) There’s a tiny microphone on the outside of the earphones/headphones. It picks up the external noise that’s going into the ear and copies it, but with an important difference. When the air squiggles up, the speaker plays a down, and vice versa. Basically, it plays an upside-down version of the same noise. When this goes into your ear, the ups from outside cancel with the downs from the speakers, meaning there’s no overall motion.
Sound is basically air movement at different speeds. Sound displace like a 3d bubble similar to the ripple you get when you throw a rock in a lake. If you apply the same force but completely opposite, it will cancel out. Think of it as two people blowing with the same strength a piece of paper but opposite one to each other. The pape sheet wouldn’t move. Noice cancelling has a little mic who listens and inverts the direction (phase) of the sound clashing it with each other getting the same effect. It is pretty magical
Any noise cancellation device for upstairs neighbor’s noise? Now that would be a huge market.
Sound is caused by very fast changes in air pressure. When you clap your hands, you compress the air and briefly create an area of high pressure. Right after you clap, that high pressure air wants to get back to normal pressure, so it expands out, causing a ripple effect in the surrounding air, and those air pressure ripples eventually expand out and travel to your ear. Your ear is designed to detect air pressure differences and interpret them as sounds.
Air pressure variations can be positive or negative, meaning they can either be higher-than-normal pressure or lower-than-normal pressure. Most sounds are actually a very quick oscillation back and forth between high and low pressure. For instance, if you sing a note, your vocal cords are vibrating hundreds of times per second, and that creates a small air pressure variation that varies between high pressure and low pressure several hundred times per second.
If you have two different sounds happening at the same time, their air pressure differences add together, since a region of air can’t be at two different pressures at the same time. In some cases, if the air pressure caused by one sound is positive at a certain moment, and the air pressure caused by a different sound is negative at the same moment (and in the same physical location), those two air pressures cancel out and you’re left with no change in air pressure. This is referred to as destructive interference. You can see the same thing in pond ripples. If you throw two rocks into a pond in different locations and watch the ripples spread out, when the ripples intersect there will be some small areas in the intersection where the ripples become twice as high or twice as low (due to constructive interference), and other areas where the water surface doesn’t move at all (due to destructive interference) despite all these waves passing through each other.
Noise cancelling headphones work by using this principle. They have a microphone that can record the ambient noise around you. They quickly process this noise and play the inverse of that noise through your headphone speakers, in such a way that the noise is cancelled out and you no longer hear it. Additionally, the fact that (many of) these headphones cover your ear entirely and seal it off helps to reduce the amount of noise that makes it to your ear. Furthermore, if you’re actively listening to something in the headphones like music or movies, that drowns out outside noises even more, because the music/movie sound is probably being played more loudly than the ambient noise level around you. All these things combine to reduce noise to a level where it is no longer noticeable.
The tech is indeed amazing.
I had a pair of Audio-Technica over-the-ear headphones about 12 years ago I took with me when I worked on a cruise ship. One night, I had the headphones on and the curtain to my bunk closed. Crew rooms are *very* small and to my surprise, the orchestra’s standup bass player and trumpet player (my roommate) had been practicing for the last hour without me ever having a clue.
Yea so basically it has microphones that listen to what you are hearing right now, and then it sends the exact opposite of the sound wave to the speaker, so for example if the sound you are hearing has a frequency (pitch) of 500hz at an amplitude (loudness) of 30db it would send a signal of 500hz at -30db (so basically the same sound wave but flipped) to the speaker, where the two waves collide and cancel out (destroy) eachother (this is why it is called noise cancelling), and this is also why bad implementations of it can cause a high pressure in your ear, making you feel uncomfortable, yes it will drain the battery considerably
When two sound waves happen at the same time, their magnitude adds together. I’ll try to draw a picture in ascii art.
Let’s say you have a wave x like so:
xxxx xxxx
xxx xxx xxx xxx
xx xx xx xx
x…………..x…………….x…………..x…………….
x x x
xx xx xx
xxx xxx xxx
xxxx xxxx
and a wave y, which is the same but slightly quieter, like so:
yyyyyy yyyyyy
yyy yyy yyy yyy
yy…………yy…………….yy…………yy……………..
yy yy yy
yyy yyy yyy
yyyyyy yyyyyy
When both occur at the same time, they add together to form a louder wave, z like so:
zzzz zzzz
zz zz zz zz
z z z z
zz zz zz zz
z z z z
z…………….z…………….z…………….z…………….
z z z
zz zz zz
z z z
zz zz zz
zzzz zzzz
But what if the two waves didn’t overlap so nicely, and in fact one was the inverse of the other, like so:
xxxx xxxx
xxx xxx xxx xxx
xx xx xx xx
x…………..x…………….x…………..x…………….
x x x
xx xx xx
xxx xxx xxx
xxxx xxxx
yyyyyy yyyyyy
yyy yyy yyy
yy…………….yy…………yy…………….yy………….
yy yy yy yy
yyy yyy yyy yyy
yyyyyy yyyyyy
Then at each point along the way the two waves are in opposite directions. So instead of adding to a bigger wave they add to a smaller one. They “pull” the air in opposite directions, dampening each other.
The resulting wave from that would be this subdued smaller wave:
zzzzzzzz zzzzzzz
zzzzzz……..zzzzzzzz……..zzzzzzzz………zzzzzzz…………
zzzzzzzz zzzzzzzzz
Noise cancelling technology is a device that “hears” wave x, then tries to emit wave y to counter it, resulting in the subdued wave z.
So there’s a little microphone in the headphones that hears the wave that’s about to hit your ear, and immediately emits a wave in the exact opposite direction through the headphones’ speaker to dampen it.
For the question: Does it drain the battery? Well, yes, because it’s driving the speaker that produces wave y.
The irony is that to dampen louder noises, you need a more powerful speaker.
The better noise dampening headphones also have an “awareness mode” that allows “new” sounds to leak through, not dampening them until they’ve been repeating for a little while. Thus if you’re on an airplane the constant nonstop whine of the airplane’s engines gets muffled, but the “bing bong” when an announcement is coming doesn’t. It’s not that the headphones “know” that the “bing bong” is important. It’s just that they “know” the “bing bong” is different from what they were hearing before and therefore not background noise, so let it through. If the sound kept going “bing bong bing bong bing bong” over and over, then the headphones might start dampening it.
That “awareness mode” is there so that wearing noise cancelling headphones in traffic isn’t quite as dangerous as it otherwise might be. You want to let sudden changes in sound like a driver honking their horn at you to come through at full volume. (I still don’t trust this and don’t like wearing the headphones when walking in traffic, but will wear them when sitting still next to traffic – like on a bench at a bus stop.)
I’ve heard that effect with church bells ringing out 12 o’clock. The first few bongs get through the headphones at full volume, but by the 12th bong, they’re starting to get quiet.
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